Intelligence is hard or impossible to measure in most animals, but especially difficult to measure in felines, which are not pack animals and so have little interest in social hierarchies--which means they don't pay much attention to commands from us. Dogs, on the other hand, have instincts to obey the alpha in their pack--which is often a human. Cats just don't think this way. But the Abyssinian, one of the oldest known breeds of cat, is noted by much of the cat fancy community as the smartest breed.
Abyssinians can't be trained, exactly, but they will exhibit signs of learning. They can be taught to play fetch
--perhaps the only breed of cat that will do this reliably. No studies have been performed to measure the Abyssinian's intelligence, but anecdotal evidence
suggests the Abyssinian is energetic and curious, able to figure out small puzzles (like doorknobs) quickly.